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VEHICLE: Columbia/OV-102 LOCATION: Launch Pad 39B TARGET KSC LAUNCH DATE/TIME: July 22, 1999 at 12:28 a.m. EDT TARGET KSC LANDING DATE/TIME: July 26, 1999 at 11:24 p.m. EDT LAUNCH WINDOW: 46 minutes MISSION DURATION: 4 days, 23 hours CREW: Collins, Ashby, Hawley, Coleman, Tognini ORBITAL ALTITUDE and INCLINATION: 153 nautical miles/28.45 degrees NOTE: Space Shuttle Columbia's July 20 launch attempt was scrubbed at the T-7 second mark in the countdown. Following a virtually flawless countdown, the orbiter's hazardous gas detection system indicated a 640 ppm concentration of hydrogen in Columbia's aft engine compartment, more than double the allowable amount. System engineers in KSC's Firing Room No. 1 noted the indication and initiated a manual cutoff of the ground launch sequencer less than one-half second before the Shuttle's three main engines would have started. Standard safing operations followed immediately. The safety of the flight crew and orbiter were not compromised at any time. The astronauts have returned to the KSC crew quarters.

Following preliminary system and data evaluation, launch managers are confident that the hydrogen concentration indication was false and are proceeding with a 48-hour scrub turnaround plan. A complete review of the Shuttle's main propulsion system and related sensors is being conducted today, but managers have already determined that the hydrogen concentration was actually about 114 ppm. This measurement is within allowable limits. The launch has been rescheduled for July 22 at 12:28 a.m.

Because the external ignitors at Launch Pad 39B were ignited, KSC technicians must replace them over the next two days. These ignitors burn-off the hydrogen concentration outside the orbiter, near the Shuttle main engines. The Chandra X-ray Observatory will remain powered up inside the orbiter and will not be adversely affected by the scrub. Eight middeck payloads will be removed, reserviced and installed back inside the orbiter during the down period.

Weather forecasters indicate only a 10 percent chance that weather could prohibit Thursday's launch attempt. The forecast calls for scattered clouds at 3000 feet and 25,000 feet; visibility at 7 miles ; winds from the Southeast at 6 peaking to 8 knots; temperature at 80 degrees F and relative humidity at 86 percent. The only concern is the slight chance of coastal showers.